1. Field of the Invention
The present invention relates to a method of controlling a position of a cutting blade in an operation with a machine tool, and an apparatus suitable for carrying out the method. It is noted that the present invention is applicable to any kind of cutting blades. The cutting blade may be constituted by the entirety of a cutting tool, for example, where the cutting tool consists of a solid tool which is provided by a single piece, or may be constituted by a portion of a cutting tool, for example, where the cutting tool includes a main body and a cutting tip or insert. In the latter case, the cutting blade is constituted by the cutting insert, which is removably attached to the main body.
2. Discussion of Related Art
In an operation with a machine tool, a cutting blade and a workpiece are moved relative to each other, whereby the workpiece is machined or cut by the cutting blade, so as to form a final or intermediate product having desired configuration and dimension. In the operation, a relative position of the cutting blade and the workpiece has to be appropriately controlled, for surely obtaining the desired configuration and dimensions. For example, in an automatic machine tool such as a numerically controlled machine tool in which a movement device for moving at least one of the cutting tool and the workpiece is controlled in accordance with a predetermined program, the relative position of the cutting blade and the workpiece has to be accurately detected. Namely, it is necessary to accurately obtain a distance, as viewed in each of directions parallel to controlled axes of the machined tool, between a predetermined portion of the workpiece (e.g., a reference point in the workpiece which point serves as an origin of coordinates for a cutting program), and a cutting point of the cutting blade when the cutting blade and the workpiece are positioned relative to each other in a predetermined position (e.g., a “machine home position” which is a known position within a machining space of the machine tool). To this end, prior to an actual cutting operation, there is conventionally required a step in which a touch probe (i.e., a detecting prove of a touch sensor) or a cutting blade is brought into contact with an object (e.g., a master workpiece, and a reference portion of the machine tool), and the relative position of the touch probe (or the cutting blade) and the object upon contact of the touch probe (or the cutting blade) with the object is read out from a position detecting device. In the cutting operation, the relative position of the cutting blade and the workpiece is controlled by controlling the movement device on the basis of the read-out contact position which serves as a reference position.
The present invention is applied to an apparatus and a method of detecting the relative position of a cutting blade and a workpiece, by bringing the cutting blade into contact with the object, without using a touch sensor. For example, where an automatic lathe such as a NC (numerically controlled) lathe is used as the machine tool for cutting an outer circumferential surface of the workpiece, a cutting point of the cutting blade is brought into contact with a surface of the object, by moving the cutting blade and the workpiece relative to each other in a radial direction of the workpiece. When the cutting point of the cutting blade is brought into contact with the surface of the object, the position of the cutting point and that of the contacted surface of the object is coincident with each other as viewed in the radial direction. Therefore, if a relative position of the contacted surface and the workpiece is known, it is possible to accurately form the workpiece into a product having desired configuration and dimension, by controlling the relative movement of the cutting blade and the workpiece on the basis of the contact position in which the cutting point of the cutting blade is brought into contact with the contacted surface of the object.
Also where a milling machine or a machining center is used as the machine tool for cutting a workpiece with a rotary cutting tool, the workpiece can be formed into a product having desired configuration and dimension in substantially the same manner as described above. However, in a cutting operation with a machining center, it is common that the rotary cutting tool and the workpiece are both moved so that the relative movement required for the cutting operation is obtained by combination of the movements of the cutting tool and the workpiece, although there is a case where only the rotary cutting blade is moved while the workpiece is held stationary. Where the rotary cutting tool and the workpiece are both moved, the relative movement is controlled by controlling a workpiece movement device for moving a workpiece holding member (e.g. a work table) which holds the workpiece, and also a tool movement device for moving a tool holding member (e.g., a headstock) which rotatably holds a spindle into which the cutting tool is received. When the rotary cutting tool is brought into contact with an object so as to detect the relative position as the contact position, the rotary cutting tool and the object are moved toward each other with or without the rotary cutting tool being rotated. Where the rotary cutting tool is brought into contact with contacted object with the rotary cutting tool being rotated, it is possible to determine, as the contact position, the relative position in which the object is brought into contact with one of cutting points of respective cutting blades of the cutting tool which one has a lager radial distance from the axis of the cutting tool than the other cutting points. This is advantageous, particularly, in a case where a difference among the radial distances from the respective cutting points to the tool axis has been increased, for example, due to wear of the cutting points as a result of a long service of the cutting tool.
Where the touch sensor is used for obtaining the above-described contact position, a required cost for the apparatus is increased due to expensiveness of the touch sensor itself. Further, the use of the touch sensor is likely to cause a reduction in accuracy of positioning of the cutting blade unless a positional relationship between the touch sensor and the cutting blade is accurately known. These problems could be resolved by using the cutting blade in place of the touch sensor. However, the use of the cutting blade provides a risk of damaging the cutting blade, the object or holders holding the cutting blade and the object, unless the contact of the cutting blade with the object is surely detected. For detecting the contact of the cutting blade with the object, there is conventionally used an electric circuit including the cutting blade, the object and a power source which are arranged in series. When the cutting blade and the object are separated from each other, the electric circuit is open without an electric current flowing therethrough. When the cutting blade is in contact with the object, the electric circuit is closed whereby an electric current flows therethrough. In this arrangement, it is possible to momentarily detect the contact of the cutting blade with the object, and accordingly detect the contact position, by detecting a state of the power source with a detector, namely, by detecting the electric current flowing from the power source with a current detector. However, in the event of a failure of the power source or the detector, or a trouble with disconnection of lead wires of the electric circuit, the contact of the cutting blade with the object would not be detected, whereby the cutting blade and the object are further forced to each other, causing the above-described risk of damaging the cutting blade, the object or members holding the cutting blade and the object.
Even without the above-described failure or disconnection trouble, the use of the cutting blade for the contact with the object would suffer from various problems. For example, the contact of the cutting blade with the object is likely to cause “chipping” of the cutting blade, or otherwise damage or undesirably cut the object. That is, the cutting blade is likely to chip where the contact is made without rotation of the cutting blade or the object, while the object is likely to be damaged or undesirably cut where the contact is made with rotation of the cutting blade or the object. If a member having an extremely high degree of hardness is used as the object in the interest of avoiding undesirable cut of the object, the cutting blade would be worn or chipped more easily. Another problem is caused where the entirety of the cutting blade or at least the cutting edge of the cutting blade is provided by a material having a high degree of electric resistance or a material having substantially no electrical conductivity. That is, where the cutting blade is made of a ceramic material, or where the cutting edge is made of a diamond sintered body or CBN (cubic boron nitrides) sintered body, its is extremely difficult or impossible to detect the contact of the cutting blade with the object.
Further, the conventional technique for detecting the contact of the cutting blade with the object suffer from some other drawbacks, for example, where the cutting blade attached to a holder of the machine tool is a wrong cutting blade which is not a cutting blade designated in a cutting operation program. More specifically, in a case where the cutting blade is provided by a replaceable cutting insert of a cutting tool used for a lathe cutting operation, if the cutting blade is a wrong cutting blade (i.e., a wrong cutting insert), the wrong cutting blade is likely to be brought into contact with an unexpected portion of the workpiece in the lathe cutting operation, whereby the cutting blade or the workpiece could be damaged, or the workpiece could not be formed into a product having desired configuration and dimension. In a case where the cutting blade is provided by a rotary cutting tool used for a milling operation, the position of the axis of the rotary cutting tool relative to the workpiece is controlled during their relative movement in a direction perpendicular to the axis of the rotary cutting tool, i.e., in X- or Y-axis direction. Thus, in such a milling operation, if the wrong rotary cutting tool has a diameter different from that of a correct rotary cutting tool, the workpiece is likely to be cut by the wrong rotary cutting tool with a radial depth of cut which is smaller or larger than a desired value, thereby making it impossible to form the workpiece into a product having desired configuration and dimension, and also even causing a risk of damaging the rotary cutting and the workpiece. It is needless to say that the same problems would be encountered where the wrong rotary cutting tool has an axial length different from that of a correct rotary cutting tool, since the workpiece is likely to be cut by the wrong rotary cutting tool with an axial depth of cut which is different from that a desired value.